Use of fossil fuels (example, fuel oil) in gas turbines, furnaces, internal combustion engines and boilers, such as for power plants, industrial production, etc., results in the generation of flue gases comprising undesirable nitrogen oxides (NOx), usually in the form of a combination of nitric oxide (NO) and nitrogen dioxide (NO2). Under certain operating conditions the NOx level in a flue gas stream can be lowered by reacting the NOx with ammonia to produce harmless water and nitrogen as products. This reaction can occur in the presence of certain catalysts, in a process known as selective catalytic reduction (SCR).
Ammonia for SCR is typically supplied by sufficiently heating aqueous urea to form gaseous ammonia. Use of the enthalpy of sufficiently hot bypass stream of flue gases, to convert a feed of urea into ammonia gas, is also known in the art. For example:
U.S. Pat. No. 4,978,514 titled, “Combined catalytic/non-catalytic process for nitrogen oxides reduction” to Hofmann et al., discloses a “process for reducing nitrogen oxides in a combustion effluent” that “involves introducing a nitrogenous treatment agent into the effluent under conditions effective to create a treated effluent having reduced nitrogen oxides concentration such that ammonia is present in the treated effluent; and then contacting the treated effluent under conditions effective to reduce the nitrogen oxides in the effluent with a nitrogen oxides reducing catalyst.”
U.S. Pat. No. 5,139,754 titled, “Catalytic/non-catalytic combination process for nitrogen oxides reduction” to Luftglass et al., describes “A process for reducing nitrogen oxides in a combustion effluent” that “involves introducing a nitrogenous treatment agent into the effluent under conditions effective to create a treated effluent having reduced nitrogen oxides concentration such that ammonia is present in the treated effluent; and then contacting the treated effluent under conditions effective to reduce the nitrogen oxides in the effluent with a nitrogen oxides reducing catalyst.”
U.S. Pat. No. 7,090,810 titled, “Selective catalytic reduction of NOx enabled by side stream urea decomposition” to Sun et al., discloses “A preferred process arrangement” that “utilizes the enthalpy of the flue gas, which can be supplemented if need be, to convert urea (30) into ammonia for SCR. Urea (30), which decomposes at temperatures above 140° C., is injected (32) into a flue gas stream split off (28) after a heat exchanger (22), such as a primary super heater or an economizer. Ideally, the side stream would gasify the urea without need for further heating; but, when heat is required it is far less than would be needed to heat either the entire effluent (23) or the urea (30). This side stream, typically less than 3% of the flue gas, provides the required temperature and residence time for complete decomposition of urea (30). A cyclonic separator can be used to remove particulates and completely mix the reagent and flue gas. This stream can then be directed to an injection grid (37) ahead of SCR using a blower (36). The mixing with the flue gas is facilitated due to an order of magnitude higher mass of side stream compared to that injected through the AIG in a traditional ammonia-SCR process.”
U.S. Pat. No. 5,286,467 titled, “Highly efficient hybrid process for nitrogen oxides reduction” to Sun et al., describes “A process for reducing nitrogen oxides in a combustion effluent” that “involves introducing a nitrogenous treatment agent other than ammonia into the effluent to create a treated effluent having reduced nitrogen oxides concentration such that ammonia is present in the treated effluent; introducing a source of ammonia into the effluent: and contacting the treated effluent with a nitrogen oxides reducing catalyst.”